The present invention relates to a discharge lamp used for an automobile headlight, a light source for the backlight of a liquid crystal projector or the like.
A discharge lamp is provided with an arc tube having a pair of electrodes in a gas and uses light emitted by an arc discharge generated in the arc tube. In this discharge lamp, light emitted from the arc tube includes ultraviolet rays. Therefore, there was a problem in that the ultraviolet rays deteriorate the quality of various components such as a reflecting mirror, a front glass, etc., which are located in the vicinity of the discharge lamp. In order to eliminate such a problem, a discharge lamp in which an arc tube is enveloped by an outer tube containing additives capable of absorbing ultraviolet rays has been suggested. This discharge lamp is produced by inserting the arc tube into the outer tube and then fusing the end portion of the outer tube to the arc tube.
However, in the above-mentioned discharge lamp, both the outer tube and the arc tube are made of silica glass. Since the softening temperature of the outer tube is high and the same level as that of the arc tube, when the outer tube is fused to the arc tube, the arc tube also may be softened and deformed. The softening of the arc tube causes the electrodes located in the arc tube to deviate from the appropriate location, and, in turn, an arc generated between the electrodes to deviate, which may result in deteriorating the accuracy of luminous intensity distribution of the discharge lamp.
It is an object of the present invention to provide a discharge lamp achieving a high accuracy of luminous intensity distribution by inhibiting the deformation of the arc tube.
In order to achieve the above-mentioned object, a first discharge lamp of the present invention comprises an arc tube having a light-emitting portion provided with a pair of electrodes and an outer tube enveloping the light-emitting portion and at least partly fused to the arc tube. Herein, the outer tube contains silicon dioxide as a main component and further contains boron. With such a structure, the softening temperature of the outer tube can be made sufficiently lower than that of the arc tube. Thus, when the arc tube and outer tube are fused to each other, the deformation of the arc tube can be inhibited.
It is preferable in the first discharge lamp that the outer tube contains 0.12 weight % (referred to as wt. % hereinafter) or more of boron. Thus, the softening temperature of the outer tube can be adjusted to a more preferable temperature.
Furthermore, it is preferable in the first discharge lamp that the expression: wB/Dxe2x89xa6120 is satisfied, wherein wB[wt. % ] is the content of boron in the outer tube and D [mm] is the shortest distance between the inner surface of the outer tube and the external surface of the light-emitting portion. Thus, the softening temperature of the outer tube is inhibited from becoming excessively low. Furthermore, the outer tube is inhibited from deforming with the passage of the lighting time of the discharge lamp.
Still furthermore, it is preferable in the first discharge lamp that the expression: wB/L less than 1.2 is satisfied, wherein wB[wt. %] is the content of boron in the outer tube and L [mm] is the shortest distance between the tip of the electrode located in the light-emitting portion and the portion where the outer tube and the arc tube are fused to each other. Thus, the softening temperature of the outer tube is inhibited from becoming excessively low. Furthermore, the fusing portion is inhibited from deforming with the passage of the lighting time of the discharge lamp.
Still furthermore, it is preferable in the first discharge lamp that the outer tube contains 90 to 99.88 wt. % of silicon dioxide.
In order to achieve the above-mentioned object, a second discharge lamp of the present invention comprises an arc tube having a light-emitting portion provided with a pair of electrodes, and an outer tube enveloping the light-emitting portion and being at least partly fused to the arc tube. Herein, the outer tube contains silicon dioxide as a main component and further contains at least one selected from aluminum and zirconium together with boron. With such a structure, since the softening temperature of the outer tube can be lowered and the processing temperature when the arc tube and the outer tube are fused can be lowered, the deformation of the arc tube can be inhibited.
It is preferable in the second discharge lamp that the expression: (wB+2wAl+5wZr)xe2x89xa60.12 is satisfied, wherein wB [wt. %] is the content of boron, wAl, [wt. %] is the content of aluminum and wZr [wt. %] is the content of zirconium in the outer tube.
Furthermore, it is preferable in the second discharge lamp that the expression: (wB+2wAl+5wZr)/Dxe2x89xa6120 is satisfied, wherein wB [wt. %] is the content of boron, wAl [wt. %] is the content of aluminum, wZr[wt. %] is the content of zirconium in the outer tube, and D [mm] is the shortest distance between the inner surface of the outer tube and the external surface of the light-emitting portion. Thus, the outer tube can be inhibited from deforming with the passage of the lighting time of the discharge lamp.
Still furthermore, it is preferable in the second discharge lamp that the expression: (wB+2wAl+5wZr)/Lxe2x89xa61.2 is satisfied, wherein wB[wt. %] is the content of boron, wAl [wt. %] is the content of aluminum, wZr [wt. %] is the content of zirconium it the outer tube, and L [mm] is the shortest distance between the tip of the electrode located in the light-emitting portion and the portion where the outer tube and the arc tube are fused to each other. Thus, the fused portion can be inhibited from deforming with the passage of the lighting time of the discharge lamp.
Still furthermore, it is preferable in the second discharge lamp that the outer tube contains 90 to 99.88 wt. % of silicon dioxide.
It is preferable in the first and second discharge lamps that the outer tube contains no more than 0.1 wt. % of at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium.
Furthermore, it is preferable in the first and second discharge lamps that the outer tube further comprises at least one element selected from the group consisting of cerium, titanium, iron, praseodymium and europium. Thus, ultraviolet rays radiated from the discharge lamp can be reduced. Moreover, it is preferable that the content of the above-mentioned element in the outer tube is 0.01 to 1 wt. %.
Still further, it is preferable in the first and second discharge lamps that the expression: P/Dxe2x89xa62000 is satisfied, wherein P [W] is an electric power supplied to the discharge lamp and D [mm] is the shortest distance between the inner surface of the outer tube and the external surface of the light-emitting portion. Thus, the deformation of the outer tube due to the temperature increase in the outer tube during the lighting operation of the discharge lamp can be inhibited.